This invention relates to apparatus for achieving unity power factor for electrical loads, and more particularly to an ac-to-dc or dc-to-ac converter with unity power factor at the AC port. Electric utility companies are vitally concerned about characteristics of user load currents because phase displacement of the waveforms of current with respect to supplied voltage waveforms create undesirable power-transmission losses resulting in a low power factor.
The term power factor, used to characterize transmission efficiency and to indicate how well the supplied power is utilized, is defined as the ratio of actual power to the product of effective voltage and current. For maximum efficiency of transmission of AC power, the voltage and current must be in phase at both the source and the load end of the transmission line.
The maximum power factor is unity. Practical values, however, are often much lower due to reactive loads. Utility companies at times discourage large users of power from creating low power factors by assessing a rate penalty which is based upon measured power factor. In other cases, the utility company may add inverse reactance at the load to compensate for load reactance, but that approach for increasing the power factor makes use of resonant circuits tuned to the fundamental line frequency, which requires reactive components that are extremely large and heavy. Furthermore, changes of load reactance lead to decreased power factor because compensation reactance is fixed.
Some applications require conversion of AC source power to DC load power. Battery chargers and ac-to-dc converters for control of variable speed DC motors or for DC power distribution, are examples. Supplementary AC utility power is sometimes generated from DC sources, such as solar arrays, fuel cells, etc. The dc-to-ac or ac-to-dc converters degrade power transmission if unity power factor is not obtained. Conventional converters use rectifiers or switches with firing-angle delay to control voltage conversion. Such methods result in low power factor and have the added disadvantage of requiring low-frequency filtering components, which also are large and heavy.
Power transmission losses can be minimized if the power factor of the converter itself can be adjusted to unity, instead of providing compensation to the utility power line to accommodate converters with poor power factors. Some efforts to develop converters with unity power factor for such large loads as railway electric locomotives are being made. One converter operates with AC current pulses occuring at the fundamental frequency of the utility line. These pulses are symmetric with respect to peak AC voltage, but the pulsating current generates high electromagnetic interference (EMI). In a second converter, developed for a constant-voltage application, inductance in series with the AC line smooths high-frequency voltage pulses in the converter to provide low EMI and low harmonic content while maintaining high power factor. This second converter is described in a paper presented at the 1977 IEEE International Semiconductor Power Conversion Conference by Hans Kielgas and Reiner Nill titled "Converter Propulsion Systems with 3-Phase Induction Motors For Electric Traction Vehicles". In a third converter, inductance is used in the DC line as well as in the AC line to filter internal high-frequency pulses. Very little is known about this third converter, and it has not been field tested. All of these converters operate only with a fixed DC bus, and this imposes severe limitations on potential applications.
The approach of the second converter referred to above appears to have the greatest promise. A low-power transistorized ac-to-dc converter was assembled to examine the feasibility of extending applications to include adjustable DC voltage. It was found that an important restriction on this approach originates not from the controller but from the power stage configuration. DC voltage must be greater than peak AC voltage to control peak AC current. Otherwise, current can rise to the short-circuit value, limited only by the reactance of the inductor. Limiting short-circuit current with inductance is not practical in many cases because of the large inductor required. The significance of this consideration is to preclude use of the circuit in the important class of applications that require control of the AC current at low DC voltages, e.g. full range control of DC motors. A converter which removes these and other constraints, and makes possible simplified load power factor correction to unity can have significant impact on electrical power conservation, is the subject of this application.